One of the wafers for a semiconductor device is an SOI (Silicon On Insulator) wafer having a silicon layer (hereinafter also referred to as an SOI layer) formed on a buried oxide film, which is an insulator film. The SOI wafer has features such as small parasitic capacitance and high radiation resistance because an SOI layer in a substrate surface part which becomes a device fabrication region is electrically separated from the inside of a substrate by a buried insulator layer (a buried oxide film layer (BOX layer)). Therefore, the SOI wafer is expected to produce the effects such as high-speed and low-power consumption operation and prevention of a soft error, and is regarded as a promising substrate for a high-performance semiconductor device.
In many cases, the SOI wafer having a structure composed of a base wafer, a BOX layer, and an SOI layer is generally produced by a bonding method. The bonding method is a method by which an SOI wafer is produced as follows. For example, after forming a silicon oxide film on the front surface of at least one of two silicon single crystal wafers, the two wafers are brought into intimate contact with each other via the oxide film thus formed, bonding strength is increased by performing bonding heat treatment, and one of the wafers (a wafer forming an SOI layer (hereinafter a bond wafer)) is then thinned by mirror polishing or a so-called ion implantation delamination method.
In the above-described bonding method, since a coefficient of thermal expansion of a silicon single crystal forming the base wafer and the SOI layer is different from that of a silicon oxide film forming the BOX layer by an order of magnitude or more, when bonding heat treatment is performed after the two wafers are laid one on top of another via the oxide film, residual stress accumulates in the base wafer and the bond wafer, the residual stress due to the difference in coefficient of thermal expansion between these wafers and the BOX layer. At this point, when the oxide films on the bond wafer and the base wafer have the same thickness, no noticeable warpage occurs in the bonded wafer thanks to well-balanced residual stress. However, when an SOI layer is formed thereafter by thinning the bond wafer, the SOI wafer thus produced lacks in stress balance, and the front surface thereof on which the SOI layer is present is convexly curved.
When such a warped SOI wafer is used in a device fabrication process, exposure failure and adsorption failure occur, leading to a reduction in yields of device fabrication. It is for this reason that wafer makers have developed production methods for reducing warpage of an SOI wafer by a variety of techniques (for example, see JP-B-6-80624, JP-A-11-345954, and JP-A-2007-73768).
On the other hand, in recent years in particular, with the advancement of the technology to fabricate a semiconductor device, the variety thereof has been increased. As described in JP-A-2007-73768, some wafer makers produce an SOI wafer having an extremely thick buried insulator layer with a thickness of 2 μm or more or 10 μm or more, for example.
As described above, in particular, when an SOI wafer having a thick BOX layer is produced, if a thermal oxide film which will become a BOX layer is formed only on a bonding surface of the base wafer, a reaction part (a silicon oxide film) expands to about two times its initial volume as a result of oxygen forcefully entering the Si—Si bonding. As a result, the base wafer itself is convexly curved toward a side thereof on which the silicon oxide film is formed. Therefore, as a result of both surfaces (the entire surface) of the base wafer having formed thereon a silicon oxide film having the same thickness, residual stress due to cubical expansion is produced in the thick silicon oxide film formed on the base wafer. However, the stress balance is equilibrated, whereby warpage of the wafer is cancelled out. Thus, for example, when an SOI wafer having a thick BOX layer is produced, a thick silicon oxide film is formed on the entire surface of the base wafer, and, in an SOI wafer production process that follows, in most cases, a back surface oxide film of the base wafer is not removed.
At the time of shipment of an SOI wafer, in response to requests from device makers, the wafer makers ship the SOI wafer with the thick oxide film on the back surface (on the base wafer side) of the SOI wafer, or ship the SOI wafer after removing the oxide film on the back surface. When it is necessary to use an SOI wafer that suffers less warpage in device fabrication in order to alleviate exposure failure and adsorption failure of a wafer, the device makers also often perform device fabrication processing on the SOI wafer with the back surface oxide film.
However, even when warpage is ingeniously reduced by controlling balance of residual stress accumulated in each layer of an SOI wafer when an SOI wafer having a thick buried insulator film is produced, and an SOI wafer with the back surface oxide film as described above is shipped to the device maker, if the device maker forms a silicon oxide film for fabricating a device on the front surface of the SOI layer, the SOI wafer is sometimes warped greatly after thermal oxidation treatment.
This is caused by the following phenomenon. When thermal oxidation is performed on an SOI wafer having a thick back surface oxide film for reducing warpage, since an oxidation rate is proportional to the square root of the time, almost no oxide film is grown on the back surface side on which the back surface oxide film is left, and the oxide film is grown mainly on the front surface on the SOI layer side. Furthermore, high internal stress is produced in the oxide film formed on the SOI layer side of the SOI wafer due to cubical expansion occurring when the silicon oxide film is grown.